As a power conversion device which drives a permanent magnet synchronous motor (PMSM) acting as a rotating electric machine, one described in JP-A-11-75394 (Paragraphs [0011] and and FIG. 1 or the like) is known.
FIG. 5 is a diagram showing a heretofore known technology described in JP-A-11-75394. In FIG. 5, 101 is a direct current power source, 102 is a three-phase voltage source inverter formed of semiconductor switching elements and free wheel diodes, M is a permanent magnet synchronous motor, 103 is a current detector, 104 is a current detector gain, 105 is a phase-number conversion section, 106 is a restart from idling control section, 107 is a coordinate conversion section, 108 is a magnetic pole position estimation section, 109 is a speed computation section, 110 is a current control section, and 111 is a gate signal switching section.
A so-called position and speed senseless power conversion device shown in FIG. 5 enables a smooth restart of the inverter 102 in a kind of case in which a rotor is running idle, and an induced voltage is generated in the stator winding of the synchronous motor M, in a condition in which the inverter 102 is stopped and no voltage is applied to the synchronous motor M.
That is, in FIG. 5, the restart from idling control section 106 detects a condition, in which the inverter 102 is stopped and the current of the synchronous motor M is zero, from two phase components iα and iβ of a winding current output from the phase-number conversion section 105. When the restart from idling control section 106 generates a kind of gate signal which turns on at least one of the semiconductor switching elements of the inverter 102, and switches the gate signal switching section 111 to the restart from idling control section 106 side using a switching control signal s, the one switching element is turned on, and at least one phase of the stator winding of the synchronous motor M is short-circuited. In the event that the rotor is running idle at this time, a short circuit current flows through the stator winding, via the switching element turned on and a free wheel diode of another phase, due to the induced voltage of the stator winding, and a period of conduction of the short circuit current depends on a magnetic pole position and a rotating speed, meaning that it is possible to estimate the magnetic pole position and rotating speed based on the short circuit current.
As heretofore described, when the winding current is zero and the rotor is running idle, the restart from idling control section 106 operates so as to short-circuit the stator winding of the synchronous motor M and cause the short circuit current due to the induced voltage to flow. Further, the magnetic pole position estimation section 108 and the speed computation section 109 compute a magnetic pole position θ and a rotating speed ω from the short circuit current at this time, and the current control section 110 generates an initial value of a current command or the like, and controls the inverter 102, thereby executing a restart of the device.
There is a case in which the direct current voltage of the inverter 102 is insufficient when an anomaly occurs in, for example, the direct current power source 101 in this kind of power conversion device, and after that, the power conversion device is restarted, or when the commercial power source is interrupted by a stroke of lightning or the like (including an instantaneous power interruption) in a system including a rectifier power source, into which a commercial power source and a rectifier circuit are combined, in place of the direct current power source 101, and after that, the system is restarted.
In this case, it is conceivable to compensate for the lack of voltage using a separately provided standby power supply device such as an auxiliary power supply or a battery, but the whole of a control system including the standby power supply device increases in size, thus requiring a large installation space, and moreover, leading to an increasing price.
Meanwhile, JP-A-2007-17026 (Paragraphs [0024] to [0028] and FIG. 2 or the like) describes a heretofore known technology wherein a first power converter, which rectifies a commercial power source voltage and converts the rectified voltage to a direct current voltage, and a second power converter, which drives a synchronous motor with a gas engine, thus causing the synchronous motor to operate as a generator, and converts a voltage output therefrom to a direct current voltage, are connected to a common direct current bus bar, and the direct current voltage of a direct current intermediate circuit is converted to an alternating current voltage by a third power converter, and supplied to an auxiliary machine, such as a motor.
FIG. 6 is a diagram showing the heretofore known technology described in JP-A-2007-17026.
In FIG. 6, 201 is a commercial power source, 202 is a first power converter having a diode rectifier circuit 203, an electrolytic capacitor 204, and the like, 205 is a second power converter (a PWM converter) to the alternating current terminals of which a synchronous motor M is connected and which shares a direct current bus bar with the first power converter 202, 206 is a gas engine which drives the synchronous motor M, 207 is a voltage detector, and 208 is a current detector. Also, 209 is a main controller, 210 is a computing device, 211 is a drive circuit, 212 is a motor controller, 213 is a third power converter (an inverter) connected in parallel to the first and second power converters 202 and 205, 214 is an auxiliary machine, such as a motor, which is driven by the power converter 213, 215 is a DC/DC converter, 216 is a charging control device, 217 is an electrical storage device, 218 is a starter motor control circuit, and 219 is a starter motor for the gas engine 206.
In the heretofore known technology of FIG. 6, in the event that the gas engine 206 is caused to operate at high speed, and power generated by the synchronous motor M is equal to or more than a predetermined value, the generated power is converted to direct current power by the power converter 205, supplied to the power converter 213, and converted to an alternating current voltage, thus driving the auxiliary machine 214. Also, a surplus of the direct current power is input into the DC/DC converter 215, and converted to a predetermined level of direct current voltage, with which the electrical storage device 217 is charged by the charging control device 216. The power of the electrical storage device 217 is used to drive the starter motor 219 via the starter motor control circuit 218.
Furthermore, when power generation is inefficient as when causing the gas engine 206 to operate at low speed, and power generated by the synchronous motor M is small, no power generation control is carried out, and direct current power obtained by rectifying the commercial power source 201 with the first power converter 202 is supplied to the power converter 213 and DC/DC converter 215, thus carrying out a drive of the auxiliary machine 214 and a charging of the electrical storage device 217.
According to the heretofore known technology of FIG. 6, the heretofore described operation enables an efficient operation of the auxiliary machine 214 in accordance with an operation condition of the gas engine 206.
In the heretofore known technology shown in FIG. 6, when the power generated by the synchronous motor M is equal to or more than the predetermined value, it is possible to supply power to the auxiliary machine 214 even in the event that the commercial power source 201 is interrupted, but in this case, there is the following kind of problem.
Firstly, the rotating speed of the synchronous motor M (that is, the rotating speed of the gas engine 206) in FIG. 6 can be detected by the same principle as in JP-A-11-75394. That is, when a lower-arm switching element of one phase of the power converter 205 is turned on, and in the event that an induced voltage (which shall be synonymous with a terminal voltage by ignoring internal resistance) of the synchronous motor M in the one phase is higher than in any other phase, a short circuit current flows via a lower-arm free wheel diode of a phase with a low induced voltage. As a period in which the short circuit current flows is a period of an electrical angel of 120 degrees wherein the induced voltage in the phase, the switching element of which is turned on, is higher than in any other phase, it is possible to detect the rotating speed based on a time for which the short circuit current flows or does not flow.
However, in a condition in which a direct current intermediate voltage (a voltage across the electrolytic capacitor 204) is insufficient due to an interruption of the commercial power source 201, or the like, and the induced voltage of the synchronous motor M is higher than the direct current intermediate voltage, the short circuit current of a stator winding flows via the free wheel diodes in the power converter 205 and the electrolytic capacitor 204 in the direct current intermediate circuit even though no switching element of the power converter 205 is turned on.
That is, when the induced voltage is higher than the direct current intermediate voltage of the power converter 205, the short circuit current flows uncontrollably at an unexpected timing, meaning that the short circuit current cannot be distinguished from a short circuit current caused to flow by intentionally turning on a switching element when detecting the speed. Therefore, there has been the problem that it is difficult to accurately detect the rotating speed of the synchronous motor M, or it is not possible to detect the rotating speed, the problem of a failure in starting due to a false detection of the speed, the problem that it is not possible to stably start the synchronous motor M, or the like.
Therefore, a problem to be solved by the invention is to provide a power conversion device wherein it is possible to accurately detect the speed of a rotating electric machine and stably start the rotating electric machine even when the direct current voltage of a power conversion section, such as an inverter section, is lower than the induced voltage of the rotating electric machine, and a method of controlling the power conversion device.